The proof sheets of this volume, like those of the last volume, have been looked through by Dr. Hooker and Prof. Huxley; and I have, as before, to thank them for their valuable criticisms, and for the trouble they have taken in checking the numerous statements of fact on which the arguments proceed. The consciousness that their many duties render time extremely precious to them, makes me feel how heavy is my obligation. Part IV., with which this volume commences, contains numerous figures. Nearly one half of them are repetitions, mostly altered in scale and simplified in execution, of figures, or parts of figures, contained in the works of various Botanists and Zoologists. Among the authors whom I have laid under contribution, I may name Berkeley, Carpenter, Cuvier, Green, Harvey, Hooker, Huxley, Milne-Edwards, Ralfs, Smith. The remaining figures, numbering 150, are from original sketches and diagrams. The successive instalments which compose this volume, were issued to the Subscribers at the following dates:--No. 13 (pp. 1–80) in January, 1865; No. 14 (pp. 81–160) in June, 1865; No. 15 (pp. 161–240) in December, 1865; No. 16 (pp. 241–320) in June, 1866; No. 17 (pp. 321–400) in November, 1866; and No. 18 (pp. 401–566) in March, 1867. LONDON, _March 23rd, 1867_. CONTENTS OF VOL. II. PART IV.--MORPHOLOGICAL DEVELOPMENT. CHAP. PAGE I.--THE PROBLEMS OF MORPHOLOGY 3 II.--THE MORPHOLOGICAL COMPOSITION OF PLANTS 17 III.--THE MORPHOLOGICAL COMPOSITION OF PLANTS--_Continued_ 37 IV.--THE MORPHOLOGICAL COMPOSITION OF ANIMALS 85 V.--THE MORPHOLOGICAL COMPOSITION OF ANIMALS--_Continued_ 111 VI.--MORPHOLOGICAL DIFFERENTIATION IN PLANTS 128 VII.--THE GENERAL SHAPES OF PLANTS 134 VIII.--THE SHAPES OF BRANCHES 145 IX.--THE SHAPES OF LEAVES 152 X.--THE SHAPES OF FLOWERS 161 XI.--THE SHAPES OF VEGETAL CELLS 175 XII.--CHANGES OF SHAPE OTHERWISE CAUSED 178 XIII.--MORPHOLOGICAL DIFFERENTIATION IN ANIMALS 183 XIV.--THE GENERAL SHAPES OF ANIMALS 186 XV.--THE SHAPES OF VERTEBRATE SKELETONS 209 XVI.--THE SHAPES OF ANIMAL CELLS 228 XVII.--SUMMARY OF MORPHOLOGICAL DEVELOPMENT 231 PART V.--PHYSIOLOGICAL DEVELOPMENT. I.--THE PROBLEMS OF PHYSIOLOGY 239 II.--DIFFERENTIATIONS BETWEEN THE OUTER AND INNER TISSUES OF PLANTS 244